Technologies based upon semiconductor materials and devices have a remarkable track record of commercial achievement. Silicon-based solid state electronics have given us computing technology that has doubled in performance every two years (Moore's Law) for over forty years. Additionally, compound semiconductor optoelectronics, mostly Gallium Arsenide (GaAs) and Indium Phosphide (InP) based III-V semiconductor laser diodes, have given us communications technology that doubles the data coming out of an optical fiber every nine months (Butter's Law of Photonics). Semiconductor technology is now being applied to energy and energy efficiency. Solar cell devices based on silicon and other semiconductor materials have recently experienced significant commercial success. However, it is widely recognized that there is little room left for significant improvement in solar cell power production efficiencies. In other words, there appears to be no equivalent opportunity for a Moore's Law type of improvement with solar cells. By contrast, thermoelectric materials for power generation from heat sources are increasingly being recognized as having the potential for a Moore's Law type of sustained performance improvement in the clean technology area.
In general, thermoelectric materials can be used to form thermoelectric generators and thermoelectric coolers. More specifically, FIG. 1A illustrates a traditional two-leg thermoelectric generator (TEG) 10. As illustrated, the TEG 10 includes a bulk-shaped N-type thermoelectric material 12, a bulk-shaped P-type thermoelectric material 14, a top conductive metal layer 16, and a bottom conductive metal layer 18. In order to generate power, heat is applied to the top conductive metal layer 16, thereby creating a heat differential between the top conductive metal layer 16 and the bottom conductive metal layer 18. This heat differential induces electrical current flow in the TEG 10 as illustrated. The electrical current flow through the N-type thermoelectric material 12 and the P-type thermoelectric material 14 is parallel to the direction of heat transference in the TEG 10. The induced electrical current flow supplies power to a resistive load 20.
FIG. 1B illustrates a traditional two-leg thermoelectric cooler (TEC) 22. Like the TEG 10, the TEC 22 includes a bulk-shaped N-type thermoelectric material 24, a bulk-shaped P-type thermoelectric material 26, a top conductive metal layer 28, and a bottom conductive metal layer 30. In order to effect thermoelectric cooling, an electrical current is applied to the TEC 22 as shown. The direction of current transference in the N-type thermoelectric material 24 and the P-type thermoelectric material 26 is parallel to the direction of heat transference in the TEC 22. As a result, cooling occurs at the top conductive metal layer 28 by absorbing heat at the top surface of the TEC 22 and releasing heat at the bottom surface of the TEC 22.
The primary figure of merit for thermoelectric materials is ZT, where ZT is defined as:ZT=S2σT/k, where S is the Seebeck coefficient of the thermoelectric material, σ is the electrical conductivity of the thermoelectric material, k is the thermal conductivity of the thermoelectric material, and T is the temperature in Kelvins. Thus, a good thermoelectric material will have low thermal conductivity, high electrical conductivity, and a high Seebeck coefficient. Presently, commercial thermoelectric materials have ZT values of around 1.0. However, ZT values of 3.0 or higher are desired. As such, there is a need for a thermoelectric material having a high ZT value.